Immunotherapies can reduce side reactions that can't be avoided easily in most chemotherapies, and is expected as a therapy with extremely high specificity. The immunotherapies can be executed in the purpose to recover patient's QOL by activating the immune reaction that human originally has by an extrinsic method and subrogating a part of the load by medication.
Immunopotentiation can be executed by methods of activating immune reaction of T lymphocytes. It is said that not only stimulation through antigen receptors (TCR) but also an additionally stimulative inducement through conjugated stimulative molecular groups (for example, CD28) could be necessary for the activation of T cells. However, it is reported that as the molecular groups with homologous structures to the conjugated stimulative molecular groups, CTLA-4 and PD-1 were discovered recently and give signals that suppress signals of antigen receptors (TCR). It is thought that a method of activating T cells could be an effective mean to suprress the function of those conjugated control molecules.
PD-1 was cloned as 55 kD of I type membrane protein that belong to an immunoglobulin family (The EMBO Journal (1992), vol. 11, issue 11, p. 3887-3895, JP5336973, JP7291996). Human PD-1 cDNA is composed of the base sequence shown in EMBL/GenBank Acc. No.NM—005018 and mouse PD-1 cDNA is composed of the base sequence shown in Acc. No.X67914, and those expression are observed when thymus cells differentiate from CD4-CD8− cell into CD4+ CD8+ cell (International Immunology (1996), vol. 18, issue 5, p. 773-780, Journal of Experimental Medicine (2000), vol. 191, issue 5, p. 891-898). It is reported that PD-1 expression in periphery is observed in myeloid cells including T cells or B lymphocytes activated by stimulation from antigen receptors, or activated macrophages (International Immunology (1996), vol. 18, issue 5, p. 765-772).
In an intracellular domain of PD-1, there are ITIM motifs (Immunoreceptor Tyrosine-based Inhibitory Motif) that could been thought to be a repression domain to immune reaction. Since PD-1-deficient mice develop lupus-like autoimmune disease such as glomerular nephritis and arthritis (for C57BL/6 gene background) (International Immunology (1998), vol. 10, issue 5502, p. 1563-1572, Immunity (1999), vol. 11, issue 2, p. 141-151) and a disease like dilated cardiomyopathy (for BALB/c gene background ) (Science (2001), vol. 291, issue 5502, p. 319-332), it has been suggested that PD-1 could be a control factor of development of autoimmune disease, especially the peripheral self-tolerance.
PD-L1 (human PD-L1 cDNA is composed of the base sequence shown by EMBL/GenBank Acc. No. AF233516 and mouse PD-L1 cDNA is composed of the base sequence shown by NM—021893) that is a ligand of PD-1 is expressed in so-called antigen-presenting cells such as activated monocytes and dendritic cells (Journal of Experimental Medicine (2000), vol. 19, issue 7, p 1027-1034). These cells present interaction molecules that induce a variety of immuno-inductive signals to T lymphocytes, and PD-L1 is one of these molecules that induce the inhibitory signal by PD-1. It has been revealed that PD-L1 ligand stimulation suppressed the activation (cellular proliferation and induction of various cytokine production) of PD-1 expressing T lymphocytes. PD-L1 expression has been confirmed in not only immunocompetent cells but also a certain kind of tumor cell lines (cell lines derived from monocytic leukemia, cell lines derived from mast cells, cell lines derived from hepatic carcinomas, cell lines derived from neuroblasts, and cell lines derived from breast carcinomas) (Nature Immunology (2001), vol. 2, issue 3, p. 261-267).
Though PD-L2 (human PD-L2 cDNA is composed of the base sequence shown by EMBL/GenBank Acc. No. NM—025239 and mouse PD-L2 cDNA is composed of the base sequence shown by NM—021896) had been identified as a second ligand of PD-1, it has been reported that the expression and function are almost same as PD-L1 (Nature Immunology (2001), vol. 2, issue 3, p. 261-267).
It has been thought that the inhibitory signals from the conjugated suppressive molecules represented by PD-1 could control abnormal immune reaction to autoantigen and immunological tolerance in lymphocyte generation or maturation by a mechanism that appropriately controls positive signals with antigen receptors (TCR) and conjugated stimulation molecules. It has been thought that a certain kind of tumour and virus could use those conjugated suppressive molecules to intercept the activation and proliferation of T cells and weaken the host immunity reaction to oneself by a direct or indirect mechanism (Cell (1992), vol. 71, issue 7, p. 1093-1102, Science (1993), vol. 259, issue 5093, p. 368-370). It has been thought that those conjugated suppressive molecules could have caused the impairment of T cells in a part of disease thought to originate in impairment of T cells.